Method to Liquefy Ammonia Gas

ABSTRACT

An ammonia expander in a liquefaction process, the ammonia expander reducing the resulting vapor from the liquid ammonia expansion and increasing the percentage of both the feed gas input and the process output. Embodiments of the ammonia expander comprise a radial inflow reaction turbine with an induction generator mounted on a generator shaft and a turbine mounted on a turbine shaft, the turbine submerged in the liquid ammonia. The turbine extracts torque from the liquid ammonia as it flows through the turbine. The torque from the turbine is transferred from the turbine shaft to the generator shaft, allowing for energy to be extracted from the liquid ammonia stream.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a non-provisional patent application, taking priority from provisional patent application Ser. No. 61/214,515, filed Apr. 24, 2009, and incorporated herein by reference.

BRIEF DESCRIPTION OF THE INVENTION

An ammonia expander in a liquefaction process, the ammonia expander reducing the resulting vapor from the liquid ammonia expansion and increasing the percentage of both the feed gas input and the process output. Embodiments include an ammonia expander having a radial inflow reaction turbine with an induction generator mounted on a generator shaft and a turbine mounted on a turbine shaft, the turbine submerged in the liquid ammonia. The turbine extracts torque from the liquid ammonia as it flows through the turbine. The torque from the turbine is transferred from the turbine shaft to the generator shaft, allowing for energy to be extracted from the liquid ammonia stream.

STATEMENTS AS TO THE RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.

Not applicable.

BACKGROUND OF THE INVENTION

Cryogenic liquids are refrigerated liquefied gases with boiling points below −90° C. at atmospheric pressure. Different cryogens become liquids under different conditions of temperature and pressure. Industrial facilities that produce, store, transport and utilize such gases make use of a variety of valves, pumps and expanders to move, control and process the liquids and gases. For example, Joule-Thomson (J-T) expansion valves are frequently used to reduce pressure within a system carrying liquefied natural gas (LNG). While J-T valves are important, they have limited value in comparison to certain types of liquefied gas expanders, which are able to reduce pressure while also reducing the enthalpy of the natural gas and generating work. For example, turbine expanders are able to reduce pressure and create rotational momentum that generates shaft torque (which reduces enthalpy). The shaft torque is then used by a generator to produce electrical power.

Hence, turbine expanders are frequently used to expand liquefied gas from a high pressure to a low pressure, while capturing energy generated by the expansion. In this manner, single-phase LNG expanders are used to enhance the performance of LNG liquefaction plants. Two-phase LNG expanders are further used to reduce liquefaction costs and increase production, which has the positive benefit of extending the lifetime of depleting gas fields by generating more usable liquid from the field.

Ammonia is a difficult liquid to handle because it is very hazardous to personnel if the gas is present. In addition, ammonia is very flammable, it is difficult to seal, it is highly corrosive to copper and copper alloys, and has a large affinity for water, which can make it highly conductive. Thus, while expanders have been frequently used to expand liquefied gas, they have not been used to liquefy ammonia.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1A illustrates the liquefaction process of ammonia in existing plants according to the prior art;

FIG. 1B illustrates the liquefaction process of ammonia with an ammonia expander in existing plants in accordance with an embodiment;

FIG. 2A illustrates the liquefaction process of ammonia in new plants according to the prior art;

FIG. 2B illustrates the liquefaction process of ammonia with an ammonia expander for new plants in accordance with an embodiment;

FIG. 3A illustrates a single-phase ammonia expander submerged in a vessel;

FIG. 3B illustrates a two-phase ammonia expander submerged in a vessel; and

FIG. 4 illustrates a diagram of the hydraulics for a two-phase ammonia expander.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention comprise the use of an ammonia expander in the liquefaction process of ammonia. Embodiments of the ammonia expander may comprise a submersible, magnetic coupling design. The ammonia expander can use a sealed stainless steel casing to house the generator. The generator housing has inert nitrogen gas fed into the casing from the outside of the expander vessel or tank to keep the generator section dry and separated from the ammonia. The nitrogen is pressurized to prevent ammonia from entering the casing and also to provide the proper differential pressure across the air membrane of the magnetic coupling 310. An alternative inert or mostly inert gas may also be used in place of nitrogen gas.

Embodiments of the ammonia expander have several safety advantages. The use of a submerged design means that the generator is not in the hazardous area. Since no rotating seals are used, there is no danger of leakage of flammable gas into the atmosphere. There is no need for an explosion proof generator, such as an explosion-proof housing around the generator, since the interior of the tank is under operating conditions that do not have oxygen present. Finally, embodiments are low noise since the expander and the generator are surrounded in fluid and in the insulated suction vessel.

Embodiments use an ammonia expander in the liquefaction process of ammonia. The ammonia expander replaces a traditional JT valve for more efficient process operation. The use of the ammonia expander increases the amount of liquefied ammonia and improves the overall system efficiency while recovering electric power. Embodiments increase the overall process efficiency by about 4% to about 7%. In addition, replacing the JT valve with the ammonia expander in an existing plant can result in the same volumetric flow being achieved with about a 5% reduction in compression and about a 5% reduction in overall liquefaction energy.

The key difference between a JT valve and an expander is that the expander reduces the vapor resulting from the liquid expansion. This reduction in vapor output decreases the required re-compression which in turn increases the percentage of both the feed gas input and the process output, all while recovering energy. The replacement of JT valves with an expander has the additional benefit of not requiring other plant size or capacity changes to existing plants.

FIG. 1A shows the liquefaction process in existing plants according to the prior art. A 90% feed gas line is combined with a 10% recompressed gas line and fed as input to the liquefaction process. The liquefaction process converts the gas input into 100% liquid ammonia. The liquid ammonia output is fed through a JT valve, where the liquid pressure is dropped to allow the liquid to go into another stage of compression. The output from the JT valve results in 90% liquid ammonia and 10% ammonia vapor.

FIG. 1B shows how embodiments of the ammonia expander improve the liquefaction process of ammonia in existing plants by replacing the JT valve with the ammonia expander. The ammonia expander increases the output of the liquefaction process, improving the efficiency of the plant. In addition, the ammonia expander reduces the amount of ammonia vapor generated and the amount of energy that must be used to recompress the ammonia vapor. In addition, the expander can extract energy from the liquid ammonia flow that can be fed back into the electrical system of the plant. The manner in which energy is extracted by the expander is described in reference to FIGS. 3 and 4. In relation to FIG. 1B, 95% feed gas is combined with 5% recompressed gas and fed as input to liquefaction. The output of the liquefaction is 100% liquid ammonia, which is subsequently fed into an ammonia expander. The ammonia expander outputs 95% liquid ammonia. This is in contrast to the output of 90% liquid ammonia resulting with the use of a JT valve. The 5% output vapor is then fed through a compressor resulting in 5% recompressed gas.

FIG. 2A shows the ammonia liquefaction process in new plants using a JT valve according to the prior art. A 100% feed gas line and a 10% recompressed gas are fed into liquefaction. The total gas input equals 110% to ensure a 100% liquid ammonia output. Liquefaction converts the 110% ammonia gas input into 110% liquid ammonia. The 110% liquid is then fed through a JT valve, resulting in 100% liquid ammonia and 10% ammonia vapor. The 10% ammonia vapor is then passed through a compressor outputting 10% recompressed ammonia gas.

FIG. 2B shows how embodiments improve the ammonia liquefaction process in new plants by replacing the JT valve with an ammonia expander. The ammonia expander increases the amount of liquid ammonia output and reduces the amount of ammonia vapor that must be compressed for reuse in liquefaction. In addition, the ammonia expander can extract energy from the ammonia liquid flow that can be fed back into the plant electrical system. The figure shows the use of a 100% feed gas line and a 5% recompressed gas line fed into liquefaction. The 105% total gas goes through liquefaction, resulting in 105% liquid ammonia. The 105% liquid ammonia is fed through an expander outputting 100% liquid ammonia and 5% ammonia vapor. The 5% ammonia vapor is feed through a compressor that produces 5% recompressed ammonia gas for reuse in liquefaction.

Embodiments of the ammonia expander can be designed to be used for all liquid expansion. The ammonia expander can also be designed to expand ammonia into the saturation region where the majority of the output is vapor and a small portion of the output is liquid. From herein, the output of the ammonia expander will be referred to as “expanded ammonia.”

FIG. 3A shows an embodiment of a submerged, magnetically coupled, single phase ammonia expander 300 inside a vessel 302. The ammonia expander 300 is a radial inflow reaction turbine with an induction generator mounted on a generator shaft. The turbine is totally submerged in the liquid ammonia and it is mounted on a turbine shaft. The turbine extracts the maximum amount of torque from the liquid ammonia as it flows through the turbine. The torque from the turbine shaft is transmitted to the generator shaft through a magnetic coupling. The torque from the generator shaft runs the generator allowing for energy to be extracted from the liquid ammonia stream.

The ammonia inlet flow 304 is at the bottom of the vessel 300 and allows ammonia to flow into the wet side section 318 of the ammonia expander 300, where it is expanded and output into the vessel 302 for output from the vessel 302. In alternative embodiments, the ammonia may flow from the top of the expander to the bottom. The liquid ammonia enters the expander through the inlet 304 at a higher pressure, goes through a series of one or more runner stages 306, and is output from the expander at a lower pressure. FIGS. 3A and 3B illustrate the inlet 304 at the bottom of the expander. However, alternative embodiments can have the inlet 304 at side of the expander 300 near the runner stages 306. The expander 300 can also be designed to have more than one inlet 304, with a first inlet on one side of the expander 300 and a second inlet at another side of the expander 300.

The ammonia expander 300 illustrated in FIG. 3A is an example of an expander with three runner stages 306, but different numbers of runner stages can be used. The liquid ammonia flowing through the runner stages 306 causes the turbine shaft 308 to turn. The turbine shaft 308 is mounted on product lubricated (wet) bearings 309. The turning of the turbine shaft 308 causes a magnetic difference in the magnetic coupling 310. The magnetic coupling 310 transfers the power from turbine shaft 308 to the generator shaft 312, making the generator 314 run. The generator shaft 312 is mounted on two bearings 313. The generator 314 is an induction generator including a generator rotor 316 and a generator stator 318. Hence, the expander 300 extracts energy from the high pressure flow of liquid ammonia.

The magnetic coupling 310 consists of two matching rotating parts, one rotating part mounted on the turbine shaft 308 and one rotating part mounted on the generator shaft 312 next to each other and separated by a non-rotating membrane mounted to the generator housing. The operation of a magnetic coupling is known in the art.

Ammonia is very flammable, highly corrosive to copper and copper alloys, and is highly conductive. While it is common in expanders to submerge the hydraulics and the generator in the product being expanded because submerging a generator in the fluid serves the purpose of eliminating rotating seals, it insulates the generator to provide quiet operation, and helps cool the generator. However, due to the properties of ammonia, the generator cannot be submerged in liquid ammonia because the liquid ammonia would destroy the generator. However, even if it were possible to submerge the generator in liquid ammonia, this would introduce explosion hazards because ammonia is very flammable and conductive. In addition, because ammonia is conductive the generator of the expander would not work. As a result, expanders have not been used with ammonia.

Embodiments of the ammonia expander do not submerge the generator in the liquid ammonia. Instead, the ammonia expander is divided into a dry side section 320 and a wet side section 322. The dry side section 320 houses the generator 314, the generator shaft 312 and the first part 324 of the magnet coupling 310. In addition, the dry side section 320 is nitrogen purged to remove all oxygen, to keep the spaces on the dry side inert and free from moisture, and to maintain the proper pressure balance on both sides of the magnetic coupling 310. Other inert or mostly inert gas or fluids may also be used instead of nitrogen gas. The wet side section 322 is submerged in the liquid ammonia. The wet side section 322 houses the turbine, the turbine shaft 308, and the second part 326 of the magnet coupling 310.

The expander 300 may also use a Thrust Equalizing Mechanism (TEM) device 328 for balancing hydraulic thrust. The TEM device 320 ensures that the wet side ball bearings 309 are not subjected to axial loads within the normal operating range of the ammonia expander 300. The wet side ball bearings 309 are lubricated with the liquid ammonia. When using the liquid ammonia for lubrication, it is imperative that the axial thrust loads are balanced to prevent vaporization of the liquid ammonia in the bearings to ensure reliability. Axial force along the ammonia expander is produced by unbalanced pressure, dead-weight and liquid directional change. Self adjustment by the TEM device 320 allows the product-lubricated ball bearings 309 to operate at near-zero thrust load over the entire usable capacity range for expanding. This consequently increases the reliability of the bearings, and reduces equipment maintenance requirements. Alternative embodiments of the ammonia expander may not include the TEM device 320.

An embodiment of the ammonia expander 300 uses nozzle rings (not shown) to direct the flow of ammonia into the rotating expander runners 306, which are connected to the turbine shaft 308. The torque created by the generator 314 is used to extract energy from the liquid ammonia stream, creating the thermodynamic efficiency that produces more liquid ammonia versus ammonia vapor in the expansion process. The nozzle rings can be fixed vane nozzle rings directing the flow into Francis type radial inflow runners designed to extract the maximum amount of torque from the liquid ammonia fluid as possible. The nozzle rings with converging nozzles generate high-velocity vortex flow while the turbine runners convert the angular fluid momentum into shaft torque.

An embodiment may be enhanced using variable speed technology where the generator speed is controlled using a Variable Speed Constant Frequency (VSCF) controller, thereby providing flexibility in the operating range to optimize the efficiency of the ammonia liquefaction process.

Single-phase expanders can be used when the expanded liquid ammonia will remain liquid up to or near the discharge of the final runner. The turbine runners can be designed for a wide range of flow rates, and can be supplied with one or more nozzle and runner stages to provide a wide range of pressure reduction capabilities. For example, the ammonia expander 300 consists of three runner stages 306.

The ammonia expander 300 can use variable speed together with fixed inlet vane geometry. Variable speed allows for the efficiency of the turbine to be optimized over a wide range of flows and desired pressure reductions. It enables automatic optimization of work transfer and performance to match changes in process conditions. Variable speed is enabled by a variable speed, constant frequency (VSCF) controller, which can vary the speed of the generator and expander to precisely match process conditions. This also enables for power to be produced at constant frequency with unity power factor. Alternative embodiments may not use the VSCF controller.

FIG. 3B shows an embodiment of a two-phase ammonia expander, consisting of the ammonia expander 300 with a jet exducer 330 at the end of the runner stages 306. Two-phase expanders can be used to replace the JT valves used for two-phase expansion. The two-phase expander recovers most of the available energy from the liquid ammonia stream while further cooling the liquid and thus reducing boil off downstream and increasing liquid ammonia production. In addition, the use of a two-phase expander in the liquefaction process of ammonia increases safety, increases plant efficiency, reduces overall equipment size, reduces pollution, and requires minimal space. Two-phase expanders can operate in expansion conditions where the liquid will begin partially vaporizing within the turbine. The two-phase expander can be supplied with a variable speed or fixed speed generator and operated in the same fashion as the liquid or single phase expander.

In an alternative embodiment, the two-phase expander includes a jet exducer 330 installed at the discharge end of the ammonia expander, thus allowing vapor content of up to 100% at the discharge end.

FIG. 4 shows a diagram of an embodiment of the two-phase hydraulic assembly 400 used in the two-phase expander. The hydraulic assembly consists of a nozzle ring 402, a turbine runner 404, and a jet exducer 406. The nozzle ring 402 with converging nozzles generates high-velocity vortex flow. The turbine runner 404 converts the angular fluid momentum into shaft torque. The jet exducer 406 consists of a radial outflow turbine for power generation by two-phase expansion.

The jet exducer 406, which operates in a manner similar to a garden sprinkler, is functionally based on Bernoulli's principle. Bernoulli's principle states that when a gas or fluid flows through a convergent duct, such as the nozzle ring 402 with converging nozzles, the speed of the fluid will increase and its temperature and pressure will decrease. If the area is divergent, the speed of the fluid will slow and its temperature and pressure will increase. In the two-phase hydraulic assembly 400, the nozzle ring 402 creates a convergent duct. The jets of the jet exducer 406 add liquid ammonia at the last phase of the convergent duct, just before the divergent duct. The liquid ammonia flowing out of the rotating jet exducer 406 then flows through a divergent duct, such as a condensation cone or a draft tube.

Ammonia is a difficult liquid to handle, as described above, and consequently ammonia expanders have not been used in the liquefaction process of ammonia gas. Embodiments of the single phase ammonia expander as described herein can improve the liquefaction process of ammonia by at least about 4%, whereas the two-phase expander can improve the liquefaction process by at least about 7%.

While the present invention has been illustrated and described herein in terms of a preferred embodiment and several alternatives, it is to be understood that the techniques described herein can have a multitude of additional uses and applications. Accordingly, the invention should not be limited to just the particular description and various drawing figures contained in this specification that merely illustrate a preferred embodiment and application of the principles of the invention. 

1. A process for liquefying ammonia gas, comprising the steps of: initially liquefying a first portion of a feed gas and a first portion of a compressed gas to create a liquid ammonia output; expanding the liquid ammonia output with a liquefied ammonia gas expander to generate a first portion of liquid ammonia output and a first portion of liquid ammonia vapor output; and removing energy from the liquefied ammonia gas.
 2. An expander for use within an ammonia liquefaction plant to expand liquid ammonia, comprising: a vessel having an inlet receiving the ammonia liquid and an outlet outputting expanded ammonia; a dry side section located within the vessel isolated from the liquid ammonia and expanded ammonia including a generator having a generator shaft, the generator generating energy from rotation of the generator shaft, the dry side section purged with a mostly inert gas; and a wet side section located within the vessel and submerged within the liquid ammonia including a turbine having a turbine shaft, and a series of one or more runner stages that cause the turbine shaft to rotate as the liquid ammonia flows through the wet side section, the wet side section expanding the liquid ammonia, the turbine shaft driving the generator shaft.
 3. The expander as recited in claim 2, wherein the dry side section further includes a first part of a magnetic coupling affixed to the generator shaft, and wherein the wet side section further includes a second part of the magnetic coupling affixed to the turbine shaft, the turbine shaft rotating the second part and transferring rotational power to the first part through the magnetic coupling.
 4. The expander as recited in claim 2, wherein the wet side section further includes a set of one or more ball bearings positioned around the turbine shaft and lubricated with the liquid ammonia, and a thrust equalizing mechanism device balancing axial thrust loads on the one or more ball bearings.
 5. The expander as recited in claim 2, wherein the dry side section further includes a variable speed constant frequency controller.
 6. The expander as recited in claim 2, wherein the wet side section further includes a jet exducer further expanding the expanded ammonia.
 7. An ammonia expander submerged in liquid ammonia, comprising: a rotating turbine shaft mounted in one or more bearings in an expander housing with one or more runners, the rotating turbine shaft and the one or more bearings exposed to the liquid ammonia; a rotating generator shaft mounted in one or more bearings in a generator housing with a generator rotor; a generator stator mounted in the generator housing, the rotating generator shaft, the one or more bearings, the generator rotor and the generator stator isolated from the liquid ammonia; and a magnetic coupling with two matching rotating parts, one rotating part mounted on the turbine shaft and one rotating part mounted on the generator shaft next to each other and separated by a non-rotating membrane mounted to the generator housing. 